This document describes how to configure a remote router (Cisco 3640)
to dial the central site using an analog POTS line in the event that the Frame
Relay connection goes down. The Cisco 3640 uses dialer profiles to provide the
backup interface for the Frame Relay link. Also, a specific async interface is
set aside for backup dialout. Note that this configuration can be expanded to
include Multilink PPP, which would significantly increase the bandwidth
available for the backup connection. See the
Multilink PPP for DDR - Basic
Configuration and Verification document for more information.

This document shows how to configure a backup connection with a routing
protocol running on the async link. Careful consideration is needed before
implementing such a design as the bandwidth used by the routing protocol may
curtail the bandwidth available for data transfer. Snapshot routing or static
routes may be used across the async link, instead of a routing protocol.

The information in this document is based on the software and hardware
versions below.

Cisco IOS Software Release 12.0(7)T

Cisco 3640

Cisco AS5200 Network Access Server

Cisco 7206

The information presented in this document was created from devices in
a specific lab environment. All of the devices used in this document started
with a cleared (default) configuration. If you are working in a live network,
ensure that you understand the potential impact of any command before using
it.

It is common to provide redundant paths for WAN connections such as
Frame Relay with dial-on-demand circuits. Asynchronous modems and
circuit-switched Plain Old Telephone Service (POTS) lines are regularly used to
backup WAN interfaces. Careful planning is necessary when designing dial backup
scenarios. Factors such as traffic on the backup links, the number of links
susceptible to failure, and port capacity planning to support backup circuits
must be considered.

Three common methods available to provide backup for a WAN link are:

Backup interfaces

Dialer Watches

Floating static routes

A backup interface remains inactive until the primary link goes down.
The backup link is then activated, reestablishing the connection between the
two sites. When implementing backup interfaces for Frame Relay links,
point-to-point subinterfaces on the Frame Relay links are advantageous since
the main or multipoint interfaces may remain in an up/up state even if primary
virtual circuits (PVCs) go down, causing the backup interface to not be
activated. It is also advisable to configure Frame Relay end-to-end keepalives
on your network for Cisco IOS® Software release 12.05(T) or later. Refer to the
Frame
Relay End-to-End Keepalive document for more information.

A Dialer Watch provides reliable connectivity without relying solely on
defining interesting traffic to trigger outgoing calls at the central router.
The Dialer Watch monitors certain specific routes and, if those networks are
unreachable, the Dialer Watch brings up the secondary link. Refer to the
Configuring DDR Backup
using BRIs and Dialer Watch document for more information on dialer
watches.

Floating static routes are static routes that have an administrative
distance greater than the administrative distance of dynamic routes.
Administrative distances can be configured on a static route so that the static
route is less desirable than a dynamic route; hence, the static route is not
used when the dynamic route is available. However, if the dynamic route is
lost, the static route can take over and traffic can be sent through this
alternative route. Refer to the
Configuring ISDN Backup for
Frame Relay document for an example on how to configure backups with
floating static routes.

When designing a dial backup scenario, remember to consider factors
such as traffic patterns on the backup links, the number of links that might
fail, and the number of ports available at the central-site in a worst-case
situation.

Note: The configurations in this document are based on Cisco IOS Software
Release 12.0(7)T running on a Cisco 3640 and an AS5200 Network Access Server.
The Cisco 3640 has an NM-8AM Analog Modem Network Modules Card that allows the
router to place up to eight outgoing analog calls without using an external
modem.

The AS5200 has been configured as a regular Access Server capable of
supporting both ISDN and async dialup users as well as the backup connection.

The configuration of maui-rtr-06 (Cisco 7206) is irrelevant to the dial
backup scenario. The Cisco 7206 is only used to terminate the incoming WAN
links. The configuration is not necessary to demonstrate async backup.

show ip protocols - To display the
parameters and current state of the active routing protocol process. For more
information, see the show ip protocols information
in the
IP
Routing Protocol-Independent Commands documentation.

debug dialer events - To display debugging
information about the packets received on a dialer interface. When Dial on
Demand Routing (DDR) is enabled on the interface, information concerning the
cause of any call (called the Dialing cause) is also displayed. For more
information, see the debug dialer event information
in the
Debug
Commands documentation.

debug dialer packets - To display
debugging information about the packets received on a dialer interface. The
no form of this command disables debugging output.
For more information, see the debug dialer packets
information in the
Debug
Commands documentation.

debug ppp - To display information on
traffic and exchanges in an internetwork implementing PPP. The
no form of this command disables debugging output.

[no] debug ppp {packet | negotiation | error |
authentication}

For more information, see the debug ppp
information in the
Debug
Commands documentation.

debug isdn event - To display ISDN events
occurring on the user side (on the router) of the ISDN interface. Q.931 ISDN
events are displayed (call setup and teardown of ISDN network connections). The
no form of this command disables debugging output.

debug isdn q931 - To display information
about call setup and teardown of ISDN network connections (Layer 3) between the
local router (user side) and the network. The no
form of this command disables debugging output. For more information, see the
debug isdn q931 information in the
Debug
Commands documentation.